General Overview , Structure & Function of the MSK System

Objectives 1. Define the key functions of the MSK system. 2. Demonstrate the anatomical position, and relate anatomical terms to this position. 3. Identify the 3 primary planes/axes about which movements of the body occur. 4. Identify the 4 types of tissue that comprise the MSK system and distinguish between their structural organization. 5. Describe specific functions associated with each type of tissue, and identify factors/parameters that may influence their function. Basic Structural Components

1. Bone 2. Muscle 3. Tendons & Ligaments 4. - Cartilage (hyaline, fibro cartilage)

- Vascular system – localized blood flow - Nervous system – afferent / efferent conduction

General Functions of MSK System

1. Support 2. Protect 3. Move 4. Store

**Influenced By a Host of Variables**

Terms of Reference Anatomical Position • What is it?? – Is the universal standard posture of reference • Standing facing forward • Feet shoulder width apart • Arms at the side and palms facing forward – All reference to anatomical landmarks – All movement begins from this position. Superior / Inferior: - towards the head/foot; sometimes referred to as Cephalad/ Caudal Anterior/ Posterior - front/back; sometimes referred to as Ventral/ Dorsal Medial / Lateral - towards the midline / away from the midline Proximal / Distal: - closer / further Superficial / Deep - Close to the surface / Away from the surface Plantar / Dorsal - sole of the foot / top of the foot Palmar / Dorsal - Palm of the hand / top of the hand

Planes

• An Anatomical Plane is a definitional term used to provide orientation to the body when you are observing it as a whole or the parts. Each plane can best be described as a sheet of glass that passes through the body, dividing it into 2 sections. • In human , there are three basic planes.1. The Frontal (or coronal) plane; 2. The Sagittal (or median) plane; 3. The transverse (or horizontal) plane.

• The Sagittal (or median) plane runs through the body from front to back and divides the body into left and right Frontal Plane

• The Frontal (or coronal) plane runs through the body from left to right and divides the body anterior from posterior; • The transverse (or horizontal) plane runs through the body as a cross sectional plane, dividing the body into superior from inferior sections. transverse Axis of Movement

• Movements of the human body are described as occurring about 3 primary axes of rotation. • Determined by the that is moving. • As a general rule, each axis of rotation is named for the anatomical plane that it runs perpendicular to. Frontal Axis = shoulder flexion Longitudinal Axis = medial shoulder rotation Sagittal Axis = shoulder abduction

Types of Movement

• Described in oppositional pairs •Flexion decreases the angle between two bones •Extension increases the angle between two bones • Rule #1 Flexion is any movement towards the fetal position whereas extension is any movement away from the fetal position • Rule #2 Don’t forget rule #1

Types of Movement (cont’d) • Flexion / Extension /Hyperextension • Abduction / Adduction • Medial / Lateral Rotation • Dorsi flexion / Plantar flexion • Pronation / Supination • Eversion / Inversion Types of Movement (cont’d)

• Shoulder Elevation / Depression • Retraction / Protraction • Circumduction / Composite movements

You must ALWAYS identify joint where the movement occurs i.e. flexion of the wrist, abduction of the hip

Bone

• Among body’s hardest structures – Only dentin and enamel in teeth are harder • Highly vascular • Very dynamic and metabolically active – Excellent self repair capacity – Constantly remodeling – Can easily change its properties and configuration in response to mechanical load Bone Function • Support • Movement • Storage of RBC • Protection • Mineral storage (calcium) • Energy storage (fat) Types of Bone

• Four principal types according to shape Long bones (femur/humerus) Short bones (carpels/metacarpels) Irregular bones (vertebrae) Flat bones (skull) Sesamoid bones (knee cap)

Bone Composition

• Specialized CT • Consists of – Organic extracellular matrix – Inorganic materials (minerals that make bone hard & rigid) • Exact composition dependent upon: – Age – Location – Diet – Disease

Organic Component - Collagen • Type 1 collagen makes up 90% of extracellular matrix (ECM), and accounts for 25 – 30% of bones dry weight • Gelatinous ground substance surrounds collagen fibers and serves as a “cement like” substance that holds collagen together. Constitute 5% of ECM • Water – 85% found in ECM • Bone cells Inorganic Component - Minerals • Calcium • Phosphate • Account for 60 – 70% of bones dry weight • Embedded in fibers of collagen organized along patterns of stress • Gives bone its solid consistency

4 Types of Bone Cells • Osteoprogenitor cells: – located on the external surface of bone – Divide and proliferate to form osteoblasts • Osteoblasts – Responsible for mineralization of boney matrix – Respond to stimuli and allow bone to remodel – When completely surrounded by osteoid matrix it becomes an osteocyte • Osteocyte – Center piece of boney matrix – Synthesize and resorb boney matrix to control blood calcium levels • Osteoclasts – Phagocytic cell from bone marrow – Responsible for bone breakdown and removal Types of Bone • Cortical (Compact): – Forms outer shell of bone (cortex) – Very dense structure – Always surrounds Cancellous bone, but thickness varies depending on type of bone, age, diet, and functional requirements • Cancellous (Trabecular): – Inside of bone – Thin plates arranged in a loose mesh structure – Arranged in a concentric layers with marrow between

Bone Development

• All bones formed from mesenchyme • Types of ossification 1. Intramembranous - Bone forms directly from mesenchyme during embryonic period 2. Endochondral – Cartilaginous bone formation – Primary vs. secondary ossification centers – Diaphysis / metaphysis / epiphysis Long Bone Structure – 4 components 1. Bone Cortex  The outer rim of bone surrounding the medullar cavity (dense bone) 2. Medulla  The inner cavity with the bony shaft  Contains the bony marrow: Red (blood production) and Yellow ( fat storage) 3. Periosteum  A dense white fibrous layer surrounding the cortex of the bone.  Contains blood and nerve supply to the bone  Also has osteoblasts and osteoclasts which promote ongoing bone growth and remodeling 4. Endosteum  The inner lining of the medullary cavity  Contains osteoblasts and osteoclasts to promote ongoing bone growth and remodelling.

Bone Growth & Regeneration dependent on: • Size • Depth • Location • Maturity Various Loading Modes of Bone “Anisotropic” Nature of Bone

Strongest when stress occurs along the long axis of a bone. Therefore the direction that stress is applied determines the strength of the bone.

Point of no return – where injury occurs Fractures: (dependent upon…….) 1. Muscle contraction: - Increases bone strength - Alters the stress distribution - Decrease or eliminate tensile stress on bone by producing compressive stress

2. Viscoelastic Nature - Strength varies with the rate it is loaded - Stiffer when loads applied at high rates; but also stores more energy. - Bone is strongest when forces applied along the long axis of bone are compression or tension

Fractures: (dependent upon…….)

3. Fatigue rate determined by: - Amount of load - # of repetitions - # of repetitions in a given time period (frequency of load)

4. Bone geometry: Load to failure and stiffness are proportional: - PCSA of bone - Distribution of bone around a central axis - Length of bone - A Stress Raiser can occurs surgically when a piece of bone is removed or a screw is inserted.

Fracture Healing Bone Remodeling • Wolff’s Law – remodeling of bone is influenced and modulated by mechanical stress • Bone will alter its size, shape and structure to meet the mechanical demands placed on it. • Accomplished by either: – Gravity (influenced by body weight) – Muscle activity • Careful consideration during facture healing: – Length & method of immobilization – Impact of metal implants

Skeletal Muscle

• Most abundant tissue in body (45% of TBW) • 430 skeletal muscles found in pairs • Key movements executed by fewer than 80 on the pairs • Functions to provide strength & protection to the skeleton by distributing loads and absorbing shock; maintain body posture; move bones at joints • Performs both static & dynamic work Muscle Function

1. Joint stability 2. Joint motion 3. Postural control 4. Force absorption Skeletal Muscle Structure & Organization Muscle Fiber (Groups of Myofibrils)

Basement Satellite Cell Sarcoplasm Membrane Sarcolemma

Adapted from Alberts et al, 2002 Cytoskeleton reinforces the Dystrophin, Spectrin sarcolemma Desmin, Vimentin & holds the myofibrils in place Myofibril – Groups of Sarcomeres

Ross, 2003

Contractile & Structural Proteins Musculotendinous Unit

Series Elastic Bone Component

Parallel Elastic Component Bone

Musculotendinous Unit Series & Parallel Elastic Component

SEC = Muscle Length PEC = Muscle width Muscle Function Influenced by a Host of Factors 1. Muscle architecture 2. Type of contraction 3. Type of movement 4. Speed of contraction 5. Position of muscle 6. Other factors – Fiber type – Muscle temperature – Muscle fatigue – Pre-stretching of muscle Muscle Architecture Muscles are Built for Different Things

Lieber & Fowler, 1993 Strength vs. Flexibility

Fiber Length(FL)

Muscle Length(ML) Physiological Cross Sectional Area (PCSA)

Lieber, 2002 Types of Contraction

• Isometric (static) Muscle tension created with no change in muscle length or joint position

• Isotonic (dynamic, velocity dependent) Concentric: Muscle shortens under load Eccentric: Muscle lengthens under load

• Isokinetic (dynamic, velocity independent) Types of Movements

• Agonist / Antagonist • Prime mover / Synergistic mover • Open / Closed chain movement Speed of Contraction (Load Velocity Curve) Position of Muscle

Nordin & Frankel, 2001

Immobilized - lengthened

Control - normal resting length

Immobilized - shortened Skeletal Muscle Types Muscle Growth & Regeneration

Influencing factors: 1. Extent of myotrauma 2. Age 3. Hypertrophy vs. Atrophy 4. Lengthening vs. Shortening (impact of immobilization) Growth & Regeneration of Muscle Satellite Cell Activity

• Powers muscles ability to repair • Extent of myotrauma = extent of activation • Migrate to & proliferate in areas of fiber damage • What turns the system on?? • Only so many?? Aging Process SC activity is age dependent (not muscle dependent) • Cell population decreases with age • Cell activation decreases with age • Cell proliferation decreases with age • Sarcopenia: Age related changes in skeletal muscle • Strength loss with aging that results from changes in: – Muscle fiber size (Decreased x-sectional area) – Decreased fast muscle fibers – Decreased ability to activate motor units • Why? – Less active / Lack of satellite cells / Ineffective protein synthesis or protein degradation Hypertrophy Atrophy • Adaptation of muscle to • Adaptation of muscle to stress eg. exercise disuse eg. being in a cast • Increase in the x- • Decrease in the x-sectional sectional area of muscle area of muscle fibers fibers brought about by brought about by an an increase in number of decrease in number of myofibrils (or Sarcomere myofibrils (or Sarcomere In-Parellel) In-Parellel) • Results in an increase in • Results in an decrease in muscle bulk & strength. muscle bulk & strength.

Increase/Decrease in Muscle Length

• Length increases occur as a result of stretching / limb lengthening / or remobilization procedures • Dependent on time & type of activity (or inactivity/immobilization) • Increase/decrease of sarcomeres In-Series • Why ? Body wants muscle in a position to optimize force production – Resets the resting length of the muscle

Types of Joints 1. Fibrous - Bones are held closely together with fibrous connective tissue with little or no movement 2. Cartilaginous - Bones are connected by cartilaginous disc. 3. Synovial joints - Joints have a space (cavity) - Freely moveable (diarthrodial) - Ends of the long bones lined with hyaline cartilage - Joint has an articular (synovial) capsule - Thickenings in the capsule are called ligaments

General features of Synovial Jts. • Reciprocally shaped surfaces • Hyaline cartilage articular surfaces • Joint capsule • Ligaments = thickenings of joint capsule which connect bone to bone and provide joint stability ( note not all ligaments are extensions of the capsule) • Synovial membrane = secretes synovial fluid which lubricates the joint surfaces and delivers nutrition to articular cartilage • Accessory supportive structures  Intracapsular / extrasynovial ligaments….  Extracapsular ligaments (lateral collateral ligament..lcl)  Meniscus….a fibrocartilage disc of the knee  Bursa(e)…synovial sacs to prevent friction between muscle / bone, muscle / muscle, and bone / tendon

Synovial Joint Movement

• Described in terms of the number of possible planes of movement and the shape of the articular surfaces – Uni-axial – Bi-axial – Multi-axial

Shape of Articulations

• Plane joints  This type of joint allows for gliding movements between flat surfaces as the surfaces slide over one another. Only a limited amount of movement is allowed Shape of Articulations

• Hinge joints  These joints occur where the convex surface of one bone fits into the concave surface of another bone, so making movement possible in one plane only.

Shape of Articulations

• Ball and socket  These joints are formed where the rounded head of one bone fits into the hollow, cup-shaped socket of another bone. Freedom of movement in all directions Types of Cartilage 1. Fibro – Transitional cartilage found at the margins of some jt. Cavities, in the joint capsules, and at insertions of ligaments/tendons into bone. – Examples: labrum, menisci, outer covering of the intervertebral discs. (also TMJ jt. – only synovial jt. that doesn’t have hyaline)

Types of Cartilage

2. Elastic – As the name implies – Found on outer ear, cartilage that makes up the eustachian tube, epiglottis, parts of larynx.

Types of Cartilage

3. Hyaline (articular) – Thin, dense, translucent, white CT – Covers ends of bones that make up synovial jts. – Specialized form of CT

• Devoid of blood vessels, lymphatic channels, and innervation • Lowest cellular density of all tissues • But……………….capable of withstanding very large loads, & commonly functions without failure during a person’s lifetime. • Functions: 1. Distribute loads on joints across a large area 2. Allows movements of opposing jt. Surfaces with minimal friction and wear.

Function • Tendons – Attach muscle to bone – Produce joint motion, maintain body posture – Serves as a dynamic restraint

• Ligaments & joint capsule – Connect bone to bone – Augment mechanical stability of the joint – Guide joint motion – Prevent excessive motion – Serve as static restraints Connective Tissue • Includes a variety of tissues with differing functional properties, but common characteristics that allow them to be grouped together.

• Connective tissue Proper – Loose CT (eg. Mucous membrane of Resp. system) – Dense CT (Irregular / Regular) • Irregular – eg. skin • Regular – eg. Tendons, ligaments, fascia

• Specialized CT – Adipose tissue – Blood – Bone – Cartilage Tendon & Ligament Composition • Dense regular CT • Consists of – Cellular component (20% of tissue volume) • Fibroblasts (few) – Extra cellular matrix (80% of tissue volume) • Water (70%) • Solids (30%) – Collagen (arranged in parallel fibers) – Elastin – Ground substance – Vessels Collagen – Basic Building Block

• Synthesized by Fibroblast cells • Type 1 collagen – Greater content in tendons than ligaments – Reducible cross links between collagen molecules bind them together in a “head to tail” pattern to form microfibrils – As collagen matures, cross links become non reducible and microfibrils band to form fibrils – Fibrils group together to form Fibers – Fibers aggregate together to form Collagen Bundles which are surrounded by endotenon • Collagen fiber organization is different in Tendons & Ligaments – Tendons: fibers have orderly parallel arrangement – thus able to handle large unidirectional loads. – Ligaments: Fibers loosely interlaced with one another to support stress from multiple directions.

Collagen Organization Tendon vs. Ligament Elastin • Interwoven with collagen • Thinner in nature, and thus more pliable – can be stretched and then recoil back to its original state • Amount of elastin determines how widely interspersed to tightly woven collagen is • Helps determine the mechanical properties of the tendon and ligaments • Very little elastin in tendons and extremity ligaments. • Large amount in spinal ligaments like ligamentum flavum

Vascularization • Both tendons and ligaments have limited vascularization • Tendons receive blood supply from: – vessels in the perimysium – Periosteal insertion – Surrounding tissues via paratenon and endotenon • Ligaments – hypovascular, receive blood supply from insertion points into bone. Viscoelastic Nature “How CT accommodates” When a load is applied and the length remains constant, the load (or tension) that the CT is under initially decreases, and then remains constant. When the applied load is held constant, the length of the CT initially increases, and then remains constant.

Examples: Stretching your hamstrings

Tendon / ligament Injury

• Withstand primarily tensile loading. • Injury during loading is dependent on: – Rate of load – Amount of load Mobilization / Immobilization • Ligament & tendon response to stress similar to bone: 1. Become stronger and stiffer in response to stress 2. Become weaker/less stiff in response to immobilization What have we learned today?? 1. The key functions of the MSK system. 2. The structrual organization and sub- classifications of the 4 types of tissue that comprise the MSK system. 3. Specific functions associated with each type of tissue, and factors/parameters that may influence their function.

Required reading: Moore, Dalley & Agur, Clinically oriented Anatomy Chapter 1, pages 19 - 36

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